Monopole antennas are commonly used in radio antenna design for mobile applications. A monopole antenna has a single radiating element. The simplest monopole antenna is the quarter-wave isotropic antenna. It comprises two elements, the first being a conductive radiating element that is usually a round structure and has an electrical length of ¼ wavelength. The second element is a ground plane element.
Quarter-wave antennas are excellent performing antennas and are the smallest resonating structures that are used when the radiating structure is straight. Unfortunately, the radiating structure length in radio frequency (hereinafter “RF”) bands now used in wireless communications can be prohibitively long for low profile enclosures. For example, the radiating element for a quarter-wave antenna operating at 2.4 Gigahertz (hereinafter “GHz”) to 2.5 GHz would be about 1.1 inches in length.
Vertically polarized antennas are often used in mobile applications, either as the portable terminal or the base station. However, currently available vertically polarized antennas such as the quarter-wave antenna, are often too large for current applications, where compactness is extremely important. For example, in a personal digital assistant, an extremely small antenna is particularly desirable.
Horizontally polarized antennas may be very low profile when antennas are etched on a radio personal computer (hereinafter “P.C.”) board (such as a PCMCIA or Compact Flash card), but suffer from attenuated performance in mobile applications due to incorrect polarity for most applications. Single and dual element (quarter-wave and dipole) horizontally polarized antennas have deep signal nulls around the antennas, even when the units being communicated with use the same polarization. Most mobile applications use vertically polarized antennas (monopoles) to eliminate nulls around the antennas.
What are needed are antennas to overcome the problems described above.